When oil is produced from poorly consolidated formations, migration of loose sand and erosion from sandstone into the wellbore is a constant problem. This migration of sand may eventually clog flow passages in the production system of the well and can erode downhole and surface equipment. In some instances, the clogging of the production system may lead to complete cessation of flow.
Sand migration is typically controlled by placement of a gravel pack around a slotted liner or a wire-wrapped screen and into perforations that extend from the wellbore into the formation. The "gravel" used in such a gravel pack is typically a sand of a size which is large enough to be kept out of the production liner or screen, but small enough to prevent migration of formation sand past the gravel pack and into the production facilities.
A gravel pack is generally placed by first inserting the liner in the portion of the wellbore to be packed. The wellbore may be either cased or uncased. A "crossover" tool is positioned on top of the liner. The crossover tool and liner are suspended from a workstring. The crossover tool allows a slurry of sand in a carrier fluid to be pumped down a workstring to the crossover tool, and then be routed through the crossover tool to the volume surrounding the liner. The sand is deposited both within this volume and the perforations. In a circulating gravel pack, some of the liquid carrier of the gravel pack slurry enters the liner where it can communicate to an annulus surrounding the workstring and be returned to the surface. The fluid not returned to the surface is forced out into the formation through the perforations. The percentage of the carrier fluid which returns to the surface is referred to as the return rate. A packer is placed above the liner in the annulus surrounding the crossover tool to prevent the slurry around the liner from bypassing the liner and communicating directly to the annulus around the workstring. After the sand is deposited around the liner and in the perforations, the crossover tool is disconnected from the liner and lifted out of the wellbore. A production tubing is then lowered into the wellbore and connected to the liner.
Sand placed in the perforations is preferably not mixed to a significant extent with the formation sand during the placement of the gravel pack. If the gravel pack sand mixes with the formation sand, the permeability of the gravel pack can be substantially reduced, thus causing a high pressure drop region in the perforations.
A slurry which is relatively viscous will carry sand into perforations effectively, and will generally flow in a laminar fashion that minimizes mixing of gravel and formation sand. But the rate at which a viscous slurry can be injected into a perforation is limited because the viscous carrier fluid will not be readily be forced into the formation. A less viscous carrier fluid such as water without a thickener is effective in packing the annulus around the liner, but is as not effective in carrying gravel into perforations.
SPE Paper No. 26543 discloses a method using water as a carrier fluid for a gravel packing operation wherein some of the disadvantages of using a low-viscosity carrier fluid are overcome by injection of the gravel slurry at a high rate and at a high pressure. The dominant mechanism for sand placement in gravel packing is said to be fluid flow, and therefore increased fluid flow into the perforations is expected to improve placement of gravel into the perforations. Injection of these gravel pack slurries at rates that cause the pressures within the perforations to exceed formation fracture pressures ensures a rapid leak-off rate from within the perforations. Fractures caused by exceeding the formation fracture pressure will be small because of rapid leak off of the fluid due to the absence from the slurry of a fluid loss control component. These small fractures do not create a large volume for which sand must be provided but provide a significant increase in surface area from which carrier fluid may penetrate into the formation. The additional surface area from which carrier fluid can penetrate into the formation permits a significant increase in the rate with which the slurry can enter the penetration.
The gravel packing method of SPE Paper No. 26543 addresses the problem of incomplete gravel packing of perforations, but in doing so creates considerable turbulence within the perforation. In an incompetent or a marginally competent formation, this turbulence can cause considerable mixing of the gravel packing sand with the formation sand. If these two sands are mixed during a gravel packing operation, the permeability of the gravel pack can be substantially reduced.
U.S. Pat. No. 5,251,699 discloses a gravel packing method wherein the carrier fluid contains a relatively small amount of a viscosifing polymer. This small amount of polymer improves transportation of solids into both the wellbore and the perforations. Slurry velocities in the annulus around the wash pipe of two to three feet per second are disclosed in this patent, but these relatively high velocities are achieved by circulation of gravel pack slurry to the surface rather than increasing the injection pressure.
Paccaloni and Tambini, "Advances in Matrix Stimulation Technology," JPT, March, 256-63 (March, 1993), discusses acid stimulation practices that include high rate and high pressure acid injection to stimulate production from wellbores. The purpose of the high rates of acid stimulation fluid is to clean formation damage from the perforations. Perforation pressures below fracturing pressures are used in these stimulations, and injection of gravel pack slurries under similar conditions is not disclosed.
It is therefore an object of the present invention to provide a method to gravel pack a wellbore that is effective to pack perforations extending from a wellbore into a formation. It is also an object to provide such a method that is accomplished without resulting in excessive mixing of formation solids with gravel pack solids.